Anionic polymerization of conjugated dienes with lithium initiators, such as sec-butyllithium, and hydrogenation of residual unsaturation has been described in many references including U.S. Pat. No. Re. 27,145 which teaches a relationship between the amount of 1,2-addition of butadiene and the glass transition temperatures of the hydrogenated butadiene polymers. The capping of living anionic polymers to form functional end groups is described in U.S. Pat. Nos. 4,417,029, 4,518,753, and 4,753,991. Of particular interest for the present invention are anionic polymers that are capped on one or more ends with hydroxyl, carboxyl, phenol, epoxy, or amine groups.
Anionic polymerization using protected functional initiators having the structure R.sup.1 R.sup.2 R.sup.3 Si--O--A'--Li is described in WO 91/12277 wherein R.sup.1, R.sup.2, and R.sup.3 are preferably alkyl, alkoxy, aryl, or alkaryl groups having from 1 to 10 carbon atoms, and A' is preferably a branched or straight chain bridging group having at least 2 carbon atoms. R.sup.1, R.sup.2, and R.sup.3 are preferably not all CH.sub.3. The bridging group (A') is most preferably a straight chain alkyl having from 3 to 10 carbon atoms. A preferred protected initiator wherein all of the R groups are methyl groups is described in U.S. Pat. No. 5,416,168. The polymers produced by these initiators are readily endcapped and hydrogenated to form anionic polymers having one or more terminal functional groups under commercially attractive conditions.
As described in U.S. Pat. No. 5,416,168, deprotection has preferably been accomplished by acid-catalyzed hydrolysis although contact with dilute aqueous base solution is also possible. One preferred process involves dissolving methane sulfonic acid in water in an alcohol and then adding this to the polymer cement (the solution/slurry/suspension of the polymer in the polymerization solvent). Under appropriate conditions, deprotection is also known to be able to be accomplished by contacting the polymer with aqueous mineral acid.
These deprotection processes are robust and are capable of achieving satisfactory results. However, the acid catalyzed method suffers from two significant deficiencies. The principle coproduct of this reaction, hexamethyldisiloxane (HMDS), is difficult to remove from the process solvent which is normally used, cyclohexane, greatly complicating solvent recycle. In addition, the extent of hydrolysis generally fails to exceed 97 percent. This appears to be an equilibrium limitation. Removal of the solvent and volatile silicon compounds, followed by addition of clean cyclohexane and a second aqueous acid contact, results in complete hydrolysis but this process is expensive and time consuming.
It can be seen that there is a need for a process that accomplishes a high degree of deprotection without the formation of difficult to remove silicon compounds. The present process provides these advantages.